1. Field of the Invention
The present invention relates to an air/fuel ratio control for an internal combustion engine. More particularly, the invention relates to a method and system for feedback control of an air/fuel ratio with high precision, on the basis of detected values of two air/fuel ratio sensors.
2. Description of the Related Art
A typical air/fuel ratio control system for an internal combustion engine, in the prior art, is disclosed in Japanese Unexamined Patent Publication No. 60-240840.
In brief, the system disclosed in the above-identified publication detects an intake air flow rate Q and an engine speed N, calculates a basic fuel supply amount Tp (=K.multidot.Q/N; K is constant) corresponding to an amount of air introduced into an engine cylinder, corrects the basic fuel supply amount Tp with correction factors, such as engine temperature and so forth, further performs a feedback correction using an air/fuel ratio correction coefficient (air/fuel ratio correction amount) set by a signal from an air/fuel ratio sensor (oxygen sensor), which detects air/fuel ratio of a mixture by detecting oxygen concentration in an exhaust gas, and performs a correction based on a battery voltage and so forth to thus set a final fuel supply amount TI.
Then, by outputting a drive pulse signal having a pulse width corresponding to the set fuel supply amount TI, to a fuel injection valve, a predetermined amount of fuel is injected to the engine.
The air/fuel ratio feedback correction based on the signal from the air/fuel ratio sensor is performed so as to control the air/fuel ratio to be near a target air/fuel ratio (stoichiometric air/fuel ratio). This is because an emission control catalyst device (catalytic converter) disposed in an exhaust system for oxidation of CO and HC (hydrocarbon) in the exhaust gas and for reducing NOx is set to operate with an optimal converting efficiency (purification efficiency) at the exhaust gas condition corresponding to combustion of the stoichiometric air/fuel ratio mixture.
The air/fuel ratio sensor is provided to swiftly vary a generated electromotive force (output voltage) in the vicinity of the stoichiometric air/fuel ratio. Therefore, by comparing the output voltage V.sub.0 with a reference voltage (threshold level) corresponding to the stoichiometric air/fuel ratio, a judgement can be made whether the air/fuel ratio of the mixture is rich or lean. For example, when the air/fuel ratio is lean (rich), the relatively large proportional component P of the air/fuel ratio feedback correction coefficient .alpha., which is to be multiplied by the basic fuel supply amount Tp, is increased (decreased) at the initial cycle after switching the air/fuel ratio to lean (rich), and is subsequently increased (decreased) by a given integral component I at every cycle to control the air/fuel ratio to be near the target air/fuel ratio (stoichiometric air/fuel ratio). It should be noted that there are some air/fuel ratio control systems which neglect the proportional component and set the air/fuel ratio feedback correction coefficient .alpha. by an integration control.
In the above-mentioned normal air/fuel ratio feedback control system, the air/fuel ratio sensor is located at the convergent section of the exhaust manifold close to the combustion chamber, to obtain higher response characteristics with a single air/fuel ratio sensor, but since the exhaust gas temperature at this portion is high, it affects the air/fuel ratio sensor to thus cause a variation of the sensor characteristics due to thermal influence or fatigue. Furthermore, the mixture of the exhaust gas from each engine cylinder is insufficient and makes it difficult to detect an average air/fuel ratio over all of the engine cylinders, and thus makes the precision of the detection of the air/fuel ratio low. This necessarily causes a degradation of the precision of the air/fuel ratio control.
In view of the above, there has been provided a system providing an additional air/fuel ratio sensor downstream of the emission control catalyst device, for performing an air/fuel ratio feedback control using two air/fuel ratio sensors. (see Japanese Unexamined Patent Publication No. 58-48756).
Namely, although the downstream side air/fuel ratio sensor has low response characteristics because it is located away from the combustion chamber, it is not significantly influenced by a balance of the exhaust gas components (CO, HC, NOx, CO.sub.2 and so forth), and is subject to a lesser amount of corrosive components in the exhaust gas to thus have less possibility of causing variations of the characteristics due to an influence of the corrosive substance, because it is located downstream of the emission control catalytic device. In addition, since the exhaust gas has a good mixing condition, a substantially average air/fuel ratio over all engine cylinders can be detected, to thus demonstrate higher accuracy and a higher stability in a detection of the air/fuel ratio.
Therefore, by combining two air/fuel ratio feedback correction coefficients respectively set based on the detected values of two air/fuel ratio sensors through the same process set forth above, or alternatively, by correcting the control constant (proportional component or integral component) of the air/fuel ratio correction coefficient set by the upstream side air/fuel ratio sensor, or correcting the comparative voltage of the output voltage or delay time of the upstream side air/fuel ratio sensor to compensate for fluctuation of the output characteristics of the upstream side air/fuel ratio sensor by the downstream side air/fuel ratio sensor, is to enable a high precision air/fuel ratio feedback control.
In the air/fuel ratio control system employing two air/fuel ratio sensors, however, it is possible to significantly vary the demand level of air/fuel ratio correction between the active state of the feedback control and inactive state of the feedback control. Particularly, at the transition from the inactive state of feedback control to the active state of feedback control, the following problem can arise at an initiation of the feedback control.
Namely, in the above-mentioned case, the feedback control speed of the downstream side air/fuel ratio sensor is set to be smaller than the feedback control speed of the upstream side air/fuel ratio sensor. That is, since the air/fuel ratio correction by the downstream side air/fuel ratio sensor is for a fine adjustment of a fluctuation of the output characteristics of the air/fuel ratio sensor of the upstream side, it may cause hunting when the feedback speed is large, but by making the feedback speed of the downstream side air/fuel ratio sensor low, it will take a long time to reach the air/fuel ratio correction amount (for example, the correction amount for the proportional component of the air/fuel ratio feedback correction coefficient by the upstream side air/fuel ratio sensor). This results in a degradation of the fuel economy, drivability, and emission control performance.
On the other hand, even during an active state of the air/fuel ratio feedback control, when the driving condition of the engine is transferred to a different range, the air/fuel ratio can be significantly offset from the target air/fuel ratio. Even in this case, the fuel economy, the drivability, and emission control performance can be degraded.
Accordingly, there has been proposed an air/fuel ratio control system in which the typical value of the second air/fuel ratio correction amount on the basis of the downstream side air/fuel ratio sensor is calculated as a learnt correction value from time-to-time and stored with respect to the respective engine driving range, and the fuel supply amount set with a correction using the learnt correction value, to provide stable air/fuel ratio control. (see Japanese Unexamined Patent Publication No. 63-97851).
On the other hand, the second air/fuel ratio correction amount based on the downstream side air/fuel ratio sensor is used to gradually correct the offset of the first air/fuel ratio correction value. Therefore, the control period of the second air/fuel ratio correction value is set to be long because a shorter control period may result in a large overshoot of the air/fuel ratio. Accordingly, when the engine driving ranges for storing the learnt correction value are divided into relatively small ranges, the period of a respective driving range becomes short. Since the control period is relatively long, learning cannot be progressed effectively.
On the other hand, the demand value of the learnt correction value is significantly differentiated depending upon the driving conditions (active or inactive states of EGR and so forth) and the basic value of the proportional component (in the case of a vehicle with a manual transmission, the proportional component for a certain driving range is set particularly small in order to avoid surge). Therefore, excessively large driving ranges for storing the learnt correction values may cause a degradation of the learning accuracy.
Accordingly, conventionally, it has been attempted to establish a balance of a quick progress of learning and an accuracy of learning to set the size of the driving ranges to store the learnt correction values, but a difficulty is encountered in satisfying both, thus causing a degradation of the exhaust emission characteristics or a degradation of the drivability due to fluctuations of the air/fuel ratio.
The present invention is intended to solve these problems in the prior art. Therefore, an object of the present invention is to satisfy both a promotion of learning and an improvement of the accuracy of learning by varying the learning speed of the learnt correction value, i.e., the modification ratio per respective learning cycle, depending upon a degree of progress of the learning.
Another object of the present invention is to provide high efficiency of reduction of emission levels of CO, HC, NOx and so forth by appropriately controlling the air/fuel ratio instantly in response to a variation of the driving range.
A further object of the invention is to maintain a proper control of the air/fuel ratio over a long period, in order to maintain a high efficiency of the reduction of the emission level.
A still further object of the invention is to restrict a difference of a degree of progress of learning between driving ranges by employing unified learning reflecting a part of a result of a learning with respect to each driving range for an overall driving range, for promoting a learning in all driving ranges.
A further object of the invention is to further promote a learning and improve an accuracy of a learning by varying the modification rate of the unified learning depending on a degree of progress of the unified learning.